An integrated circuit (IC) package can include multiple IC dies disposed on a substrate. A heat spreader can be mounted to the substrate to cover the multiple IC dies. A thermal interface material can be disposed between the IC dies and the heat spreader. An assembly process for attaching a heat spreaders to IC packages includes a step of applying a mechanical force to the heat spreaders at higher than room temperature. The IC packages can have some warpage at room temperature. The curvature of the IC packages changes when heated during the assembly process. Further, there are variations in the flatness of the heat spreader, the assembly tray supporting the IC packages, and weights used to apply force to the heat spreaders. These variations in flatness, when combined with the change in curvature of the IC packages during assembly, result in application of non-uniform force to the IC packages and non-uniform heating of the IC packages. The non-uniform application of force and heating can cause mechanical stresses in the IC packages, resulting in cracking and/or delamination. Further, the thickness of the thermal interface material will not be uniform, creating areas of higher thermal resistance in certain locations of the IC packages. Thus, it is desirable to reduce or eliminate mechanical stresses during an assembly process that integrates a heat spreader and/or stiffener with IC packages.